In 2006, the U.S. DOT initiated the UPA (Urban Partnership Agreement) program to demonstrate congestion reduction through strategies consisting of combinations of the 4Ts: Tolling, Transit, Telecommuting/TDM, and Technology. The Seattle/LWC UPA projects focused on reducing traffic congestion on SR 520 between I-405 and I-5, a heavily-traveled, east-west commuter route across Lake Washington. The lake separates Seattle from the eastern suburbs (including Bellevue and Redmond). These strategies included tolling all lanes of the SR 520 bridge, adding bus service in the SR 520 corridor, implementing active traffic management systems, and real-time traveler information signs on highways and at transit stops and stations. The suite of projects had a combined benefit-to-cost ratio of 1.76 due to travel time savings and reduced emissions.
A statistical analysis using a linear mixed-effect model was conducted to determine the level of difference between pre- and post-deployment vehicle throughput on SR 520 and I-90. The time-of-day and deployment were considered fixed effects, while monthly and day-of-week traffic patterns and other factors were treated as random effects. Using this approach, the factors that are known to potentially influence the traffic patterns are still accounted for in the models, while the fixed-effect results indicate the magnitude and statistical significance of the UPA deployment impacts.
Average peak period vehicle throughput declined on SR 520 after implementing the UPA improvements. For these periods, average peak period throughput on SR 520 declined between 25 and 33 percent in the eastbound direction and by 12 percent in the westbound direction during the a.m. Meanwhile, average peak period vehicle throughput on westbound I-90 did not change substantially in the morning peak and declined in the afternoon peak period, suggesting I-90 may have been able to absorb the shifting diverted demand from SR 520 in the a.m. peak but not during the p.m. peak.
Overall, WSDOT reported traffic volumes about 34 percent lower than pre-toll levels.